Other Assets and Liabilities - Summary of Components of Other Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Accrued interest receivable	$ 52,481	$ 38,573
Derivative assets	6,276	1,214
Loan portfolio payments held by servicer	1,549	54,759
Prepaid expenses	364	798
Prepaid taxes	28	31
Collateral deposited under derivative agreements		4,120
Other	27	80
Total	$ 60,725	$ 99,575